Apart from the adsorption of gases, vapors and odoriferous materials from gas streams and the purification of liquids, activated carbon has been used to a significant extent in catalytic processes. The catalytic characteristics of activated carbon can be improved by impregnating it with various substances.
Known impregnating agents are, for example, iodine compounds or elemental iodine. The iodine has been found to catalyze, for instance, the transformation of sulfur dioxide from waste gases into sulfuric acid (see German Auslegeschrift 1,227,434) or the oxidation of hydrogen sulfide contained in waste gases to elemental sulfur (see German Auslegeschrift 1,224,865).
The impregnation of the catalyst is generally carried out by spraying the layer of activated carbon with an iodine-containing or an iodine-compound-containing solution or by immersing the active carbon in such a solution or by flooding the active carbon layer with the solution.
A disadvantage of the conventional impregnating processes is found in the difficulty of distributing the iodine compound uniformly in the activated carbon.
Satisfactory distributions of iodine in the adsorber have only been obtained in conventional processes when the spraying or saturation of the active carbon is carried out in an agitated vessel, a rotary drum or the like. Such techniques are costly and it has long been desirous of obtaining a uniform impregnation of activated carbon with iodine compounds for the purposes described above in situ, i.e., in the adsorber or catalyst reactor in which the active carbon is to be used. In other words it is highly preferable to provide a static bed of active carbon in an adsorber or catalytic reactor and to impregnate this bed in place with the iodine compounds without agitation of the bed or other special means for accomplishing the uniform distribution. In practice, however, uniform distributions throughout the static bed with iodine-containing solutions have not been attainable heretofore.
Dissatisfaction has also been experienced with active carbon beds in adsorbers or catalytic reactors which must be after-impregnated with solutions, i.e., for regeneration of the bed, since it is not practical in such cases to use agitation techniques to obtain a uniform distribution of iodine.
The uniformity of the distribution of iodine can be determined by measuring the iodine or iodide concentration at various locations in the bed. When conventional techniques are employed with a static bed impregnated from the top, such measurements show practically all of the iodine concentrated in upper regions of the bed while the lower regions of the bed are substantially free from iodine.
Original impregnation of the static bed or after-impregnation results in a bed in which the greater proportion of the iodine is found in a limited region or layer at the inlet side for the impregnating solution while the greater part of the active carbon is free from iodine although it has been permeated with the solution.
This effect is especially noticeable when the active carbon is saturated with sulfuric acid which arises as the main product or a byproduct in many of the desulfurization processes utilizing active carbon beds. The bed may also be charged with sulfuric acid after washing since the washing operations seldom completely remove the sulfur compounds from the bed.